The invention relates to a display device having at least one electronically controllable display element. More specifically, the invention relates to a display device which makes use of the effect of electrowetting.
It is well known to realize display devices with the aid of colored liquids wherein the liquids can be displaced between positions which are not visible and those which are visible for a viewer. An efficient means for displacing liquids is the so-called electrowetting wherein a voltage is supplied to an electron array which is immediately adjacent to a liquid droplet, so that an electric field acts upon the liquid droplet, consequently increasing surface energy and therefore surface tension of the liquid. Depending on the configuration, this can lead to spreading of the liquid on the electrode or covering the same completely, depending on the geometry and/or surface nature of the electrode and on the relative position of liquid and electrode also with regard to the earth gravity field. It is known to provide a hydrophobic coating to the electrodes, in order to achieve a maximum effect between the state of the liquid when the electrode is not driven and when the electrode is driven. Using only the aforementioned means, it is already possible to realize a droplet movement without reverting to further functional elements.
By a local variation of the electric field strength, therefore the local surface energy of the liquid may be modified and thus, the geometry of a droplet can be changed locally. In particular, a movement of the droplet may occur caused by generation of a locally increased surface energy, wherein said droplet extends preferably into the corresponding region characterized by the presence of an electric field. Inversely, liquid is dragged from the influenced part of the droplet due to the surface tension, out of a region in which it is not influenced and therefore comprises a lower surface energy, since it tends in this region towards a geometry which is lowest in energy, namely that of a sphere. A net transport of liquid from the region of lower field strength to the region of higher field strength results thereby.
Various display devices have already been realized on this basis. Usually, it was always the objective to create an electronically controllable bistable display element which selectively assumes one or the other state. It is for example known to reciprocate a liquid between a first and a second volume which are in communication via a duct with the aid of the effect of electrowetting, wherein each volume comprises an associated electrode, so that the conveying direction respectively results from the ratio of field strength between both electrodes. Preferably, a potential is respectively supplied only to that electrode in which direction the liquid is to be conveyed.
A display device comprising an electronically controllable display element is known from U.S. Pat. No. 5,956,005 A, wherein an electrode array is configured to move a polar liquid from a reservoir volume into a flat subvolume, wherein said polar liquid assumes a volume which is dimensioned such that independently of a supply of voltage to the electrode array a fluidically tight closure between the reservoir volume and the flat subvolume will always form.
An electrowetting based display device is known from WO 2009/036272 A1, wherein a pair of electrodes forming a capacitor is associated with each display pixel, whereby a liquid located in the visible volume of the pixel can be charged with an electric field. A return transport of the polar or electrically conducting liquid into a reservoir which is below a visible display area in viewing direction to the display element is achieved in that the geometries of the visible display volume and of the reservoir are adjusted, so that the polar or electrically conducting liquid, if not charged with an electric field, is drawn into the reservoir due to the Young-Laplace pressure.
If a multi-colored display is to be realized with the aid of the aforementioned technology, this would be possible only by means of a plurality of pixels which are arranged side by side on said display area and which comprise differently colored liquids. However, it is not possible to operate the device according to WO 2009/036272 A1 in such a way that an individual pixel may assume more than two color states.
Moreover, the technologies known from the prior art are based on the principle that a liquid exchange between two corresponding subvolumes always takes place for a change of state of a display element, wherein the one liquid is an electrically conducting or polar liquid which, when energized by an electric field, can be conveyed, and the other liquid is a non-polar liquid which evades the electrically conducting or polar liquid precisely into that subvolume from which the electrically conducting or polar liquid is removed. In order to avoid a hydraulic bottleneck, backflow ducts are always provided, so that depending on the movement of the electrically conducting or polar liquid, the non-polar liquid can flow off for pressure equalization.
Use of backflow ducts has not only aesthetic disadvantages since they must at least partly be arranged in the visible subvolume of the display element, but furthermore also the disadvantage that they need a certain space which counter acts to the basic aim of constantly increasing pixel densities.